Power supply for electron microscopes



May 29, l945- A. w.V vANcE 2,372,283

POWER SUPPLY Foa ELEcRoN NucRossoPEsl Y original Filed April 28, 1942 s sheets-Sh't 1 May 29 w45 A. W. vANcE v 2,377,283

POWER SUPPLY FOR- ELECTRON MICROSCOPES Original Filed AprilJ-ZB, 1942 3 Sheets-Sheet 2 Gtfomeg May 29, E945. A. w. vANcE 377,283

POWER SUPPLY FOR ELECTRON MICROSCOPES Original Filed Apkril 28, 1942 3 Sheets-Sheet 5 nventor Patented May 29, 1945 POWER SUPPLY FOR ELECTRON MICROSCOPES Arthur W. Vance, Granbury, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 28, 1942, Serial No. 440,771, which is a division of application Serial No.- 365,750, November 15, 1940, now Patent No.

2,302,900, dated November 24, 1942.

Divided and this application April 17, 1943, Serial No.

6 Claims.

The subject invention relates to power supply systems for electronic devices and has for its principal object the provision of an improved method and means for producing a highly stabilized source of voltage for use with the electron microscope whereby greatly improved performance may be obtained.

This application is a division of applicants copending U. S. application, Serial Number 440,771, filed April 28, 1942, which is a division of U. S. application Serial Number 365,750 led November 15, 1940, upon which U. S. Patent 2,302,900 was granted to applicant November 24, 1942.

The development of the electron microscope has presented new problems in the design of high voltage power supplies and, in the design of current regulating systems for the various electromagnetic coils which constitute the magnetic lens system of the microscope. In order to produce a beam of high velocity electrons, an electron gun is utilized which is energized with a direct current Voltage which may be as high as 100 kilovolts. The current load of the high voltage source is of the order of l milliampere, and the problem, therefore, is one of voltage stability The mag'- netic lenses require currents of the order of 400 milliamperes which likewise must be maintained constant over short periods.

The stability of the voltage supply system for an electron microscope is measured over the period of time required to properly expose a photographic plate used to record the images. In practice, the longest exposure is approximately 30 seconds. Consequently, the Voltage supply systern` must notvary appreciably within this period,

since slight variations in the electron speed or in the focus of the magnetic fields will defocus the electron image and distort the photograph.

It is also necessary to consider the short-time constancy of the power supply system-since'this limits the resolution of the microscope. Theo-v retical calculations based on a resolving limit of 10 Angstrom units indicate that the-most critical current must not-vary more than .004 of one percent and that the high voltage should not vary more than .006 of one percent.

Itis well known that the electron beam must be thoroughly shielded from all external elds. The difficulty of shielding magnetic fields produced by 60 cycle' currents has heretofore necessitated the use of directcurrent to supply power to the filament of the high velocityelectron gun. Since the cathode is normally operated-at a high negative potential, withtheanod'e grounded, the filament supply is necessarily at a high voltage with respect to ground. It is a further object of thisinvention to eliminate the necessity of supplying direct current for the electron gun by energizing the filament with radio frequency currents. A still further object of this invention is tov provide an improved. high voltage rectifier circuit having a feedback stabilizer which permits the required high Voltage to be obtained with less equipment and fewer parts than heretofore have been required, thus greatly reducing the cost and size of the powerA supply system. Still further objects of the present invention are to provide an improved automatic relay and protective circuit; to provide an improved oscillator for supplying the radio frequency currents utilized in energizing the high voltage rectier; and to provide improved current regulators for use in conjunctionv with the microscope lenses.

Briey, these objects are accomplished by employing a source of radio frequency currents coupled to a voltage doubling rectier .circuit in which the filament of one of the rectier tubes is energizedA with radio frequency currents, thus facilitating filtering problems, deriving a potential corresponding in amplitude to the rectified output, and utilizing this potential to control the output of the oscillator supplying the high frequency currents to the rectifying system.

This invention will be better understood from the following description when considered in connection. with the accompanying drawings in which Figurel is a block diagram of theessential elements of a complete power supply system for an electron microscope; Figure 2 is an equivalent circuit diagram illustrating the operaion of the high voltage rectifier and filter system; Figure 3 is a simplified circuit diagram of the system including the high voltage rectifier; Figure 4 is the circuit diagram of the driver oscillator which supplies high Voltage radio frequency currents-to thehigh voltage rectifier; Figure 5 is the circuit diagram of a D.C. modulating amplifier; Figure 6 is a circuit diagram of a protective circuit including an overload relay; Figure 7 is'the circuit diagram of a, regulated current supply suitableifor use in'conjunction with the electromagnets of the lens system; and Figure 8 is an alternative current regulating system.

The power supply system Referring to Fig. '1, a complete power supply system for an electron microscope is illustrated in block diagram form. `Power is derived from a cycle-power line 9 and applied to a number of regulated power units which energize various components of the system and which are designed to have output voltages which are determined by their intended use. The regulated power supply ll energizes a pair of low frequency oscillators I3 and I5 which are utilized, respectively, to energize the filament of the electron microscope and the filament of one of the high voltage rectiers. The high frequency-high voltage rectifier and filter circuits are represented by the block l1 and will be described in detail hereinafter. Another power unit I9, which may be unregulated, energizes a driver oscillator 2l which generates high voltage radio frequency currents for the high voltage rectifier. A modulating amplifier 23 is energized through a connection 22 by the unregulated power unit I9 and is also supplied with a voltage over a lead 25 which is proportional to the rectiiied output voltage. This voltage is compared to that supplied to the modulated amplier by a standard battery 2l, in a circuit which will be described in detail hereinafter, to produce an output control voltage which is utilized to control the amplitude of oscillation of the driver oscillator 2l. This control connection is illustrated by lead 2S. The amplifier is also connected by a lead 2) to a regulated power source 3| which supplies a negative voltage of the order of -350 volts, the purpose of which is to be described later.

Another regulated power unit 33 supplies a direct voltage of the order of +400 volts to the driver oscillator 2l and to the modulating arnplier 23 through a lead 35. In order to achieve the highest degree of accuracy, the modulating amplifier 23 and, in addition, a current regulator 3l for the objective coil are supplied with a regulated current through connection 215i from a source 39 which is used to energize the filaments of certain of the tubes of these devices. Regulation in the current regulator 39 is achieved by utilizing the 350 volts obtained from the regulated power supply 3l by a connection 32 as a standard reference voltage, in a manner which will be described hereinafter. The pro- `iection coil and the condenser coils 4l and i3 are energized by current regulators l5 and 1, which derive power from power supplies i2 and 44, respectively, both regulators having a connection over conductor 32 to the regulated power supply 3i. A particular advantage of this system of interconnection is that one power supv ply which is carefully regulated controls the output voltages of the other powerV supply units, and control by a regulated or standard voltage is effectuated in a much more economical manner than by any other method.

The high voltage rectifier Referring now to Fig. 2, a circuit diagram of the high voltage rectier system is shown. Since an extremely high voltage is produced, the high voltage equipment has been mounted in an oil tank l which not only protects the user from harm, but also decreases the danger of accidental i'lashover or short circuits. The driver oscillator 2l is connected to a series resonant circuit comprising an inductance Lo and a capacitance Co, the latter representing the effective lumped capacity between the point 54 and ground due to the primary of transformer T2, and the rectifier tubes Vi and V2 in series with the capacitor Cl. The cathode of the rst rectifier Vi is grounded, While its anode is connected to the inductor Lo through the coupling capacitor CI and also the cathode of the second rectier V2. The anode of the second rectifier is connected to the cathode of the high velocity electron gun of the microscope and is the source of high voltage direct potential whose polarity is negative with respect to ground.

The filament of the first voltage rectifier VI is supplied from a conventional 60 cycle line, while the lament of the high voltage rectier V2 is obtained from the filament oscillator i3 which is connected to input terminals 53, the circuit including a coupling transformer T2 having an untuned secondary and a tuned primary.

Radio frequency actuation of the high voltage rectifier has a number of advantages over the use of a conventional 60 cycle alternating power source for the production of high voltages of great constancy at low current drains. For example, much smaller filter condensers are required for a given ripple output. This means that the stored energy in the filter circuit is very much less than that of the usual 60 cycle brute force filter system. The importance oi' this is that in case of a flash-over in the high voltage circuit, due to a loss of vacuum in the microscope, for example, the stored energy is not sufficient to burn up the equipment, as was the case in the earlier microscopes. Since resonant circuits are used, the ripple fed through the rectifier capacitor is sinusoidal and consequently can be resonated out to a large degree. Furthermore, by using low loss coils in the resonant circuit, an extremely high impedance may be realized which occupies but a small space and is extremely light in weight as compared to an equivalent impedance at the conventional power frequency. Also the exciting power required by such a coil is greatly reduced.

The use of radio frequency high voltage power supply also improves the operation of the output control system. When the control is operated on the low voltage input side of the rectifier, as is desirable, the speed of control is limited by the frequency of the supply. Consequently, this limitation is negligible where the supply voltage is a radio frequency voltage.

The use of radio frequency to energize the cathode or nlament of the high velocity electron gun has two important advantages. In the first place, the microscope may be readily shielded from the stray high frequency fields which are produced, and this result is aided by the fact that no 60 cycle high voltage transformers are required which have very large external elds. In the second place, the use of radio frequency eliminates the necessity for employing bulky storage batteries or their equivalent.

The operating frequency for the rectifier system is not critical. The problem is essentially that of obtaining the required output voltage with a minimum exciting power. This, vof course, requires obtaining the maximum resonant impedance of the high voltage coil. The resonant impedance is given by the formula:

where C is the distributed capacity of the coil plus the rectifier nterelectrode capacities plus other stray capacities such as capacity from the primary to the secondary of the lament transformer T2. It, is obvious that C should be kept to a minimum, and with C at a minimum, Z may only'be raised byl decreasing.- for by'increasing Q, where Q isV the well known efficiency factor:

RKI-Ro and RL and Rc are the effective series A.C. resistances of L and C, respectively. In the frequency region fromV approximately 20 kc. up to several hundred kc., the maximum Q possible in a coil of given volume is more or less independent of.; frequency; thus, the coil should, in general, operate below 50 kc.

The rectifier circuit is seen to be of the type wherein tlieinverse voltage of the half wave rectifier VI is rectified by V2, thereby charging the output capacitor C2 to a voltage nearly equal totwice the peak voltage which appears across the inductance L0. It is to be-note'd that one side of the input circuit is grounded and that no primary winding is required on the high voltage coil.

The filament transformer T2 has a low inter- Winding capacity which is preferably of low power factor. The primary and secondary windings must be spaced sufficiently to withstand the peak voltage output, and, as a result, a large amount of energy'mustbe stored in the tuned primary in order that the secondary may absorb the power necessary to excite the filament. Preferably, the tuned primary of,l transformer T2 constitutes the tank circuit of the driving filament oscillator. The-size of the secondary is selected to match the impedance of the filament load and to provide-the'desiredvoltage. The filament current may be controlled conveniently by varying the frequency of the oscillator I5.

Fig. 3 shows the rectifier circuits connected to the other elements of the power supply system. It will be noted that the negative voltage for the electron gun cathode obtained from the output of rectifier V2 is applied to a center tappedresistor 51 connected across the secondary of the transformer T3 which supplies radio frequency current to theiilament of the electron gun 59. The filament, as a whole, is 60 kilovolts below ground potential and must therefore be carefully insulated. The radio frequency current supplied bythe filament oscillator I3 is applied to the lamentI through a coaxial cable 6 I.

It Will bev noted that the filter capacitor. C2, .005 microfarad, for example, is connected to ground through a parallel connected coil L5 and condenser C6. The valuesof these reactors are such that at the operating radio frequency, effective inductive reactance of L5 and C6 resonates with capacitor C2 and forms an effective ground for ripple voltages appearingV in the output circuit. At a higher frequency, for instance, midway between the fundamental and second harmonic L5 and C6 become parallel resonant, presenting a highimpedancafbut as there is no ripple at this frequency anyway, no harm results. At higher frequencies including the second harmonic, the shunt, capacitor C6 in series with the output capacitor C2 provides an effective ground. A spark gap 63 is connectedacross the shunt reactors L5 and C6 in order to discharge voltage surges without causing a breakdown of the elements.

High voltage regulator The amplitude of the rectied direct current is controlled by a circuit including the D.C. modulating amplifier 23 which controls the driver oscillator 2 I, a voltagedivider comprising a high resistance capacity-compensated high voltage resistor165, 1100, meg., vandazrelatively low resistance resistor 61,- 10 meg., seria1ly connected'between the high voltage source and the regulator. The lower end of resistorG'I is bypassed to ground and; connected through an isolating resistor tov an adjustable tap on a standard battery 69, the nega'.- tive terminal of which is grounded; The standard voltage produced by the battery 69 is therefore compared to the divided voltage appearing across resistor''I, so that no input voltageis applied to the amplifier 23 whenthe divided voltage obtained fromthe high voltagev output is exactly equal to that of the standard battery. By Varying theftaps on the standard battery 69, thev output voltage may be controlled, for example, in 5 kv.l steps from to 60 kv.s.

Thevdetails of the driver oscillator which supplies high voltage for the rectifier tubes` VI and V2 is shown in Fig. 4. Since the resonant frequency of the series resonant inductor Lo and Co included within the oil tank 5I varies considerably with temperature, a master oscillator-power amplifier is impractical without automatic frequency control to keep it at resonance. It is therefore proposed to utilize a self-oscillating circuit whose frequency is determined wholly by the resonant frequency of the load Lo, Co. This oscillator must also be capable of modulation over a considerable range in order to provide control over the output voltage. The circuit utilized comprises a` twostage oscillator including tubes V3 and V4, the output of the latter being coupled to the input of the former by a conventional impedance coupling system, tube V4 being shunt fed through inductor L4 and coupled to the resonant' circuit load through a small series resistance R. Inductor L4 is resonated at the operating frequency by a capacitor C5. A feedback Voltage is obtained by the drop across resistor R, thel terminals of which are connected to the primary of a-shielded transformer 'II, the secondary of which is coupled to the input of the iirst oscillator tube V3. The transformer 'Il preferably has a broad frequency response, this lbeing accomplished,l for example, by a damping resistor and capacitor I3 and 15. The feedback gain is sumcient only to sustain oscillation at or very near the resonant frequency of the load Lo, Co.

The amplitude of oscillation is controlled by varying the screeny grid potential of the` output tube V4. This is illustrated in Fig, 4 by.. the potentiometer connection shown in dotted lines, the potentiometer representing the control bias derived from the D.C. modulating amplifier 23. The details ofA this connection are illustrated in Fig. 5. The tank circuit L4, C5 has a high L/C ratio and it therefore exerts only slight control over the frequency of oscillation. Its principal function is to maintain reasonably sinusoidal Voltage conditions in the plate of V4.

Referring to Fig. 5, a unique D.C. amplifier is illustrated suitable for use in applying the small Variable D.C. voltage of the control system to the driver oscillator to effectuate control of the amplitude of oscillation. Resistor 6l corresponds to the similarly numbered resistor in the voltage divider circuit of Fig. 3. At any instant, the potential of the lead 22 connecting this resistor to the control grid of the first amplifier-tube I1 is equal to the sum of the negative divided voltage and the voltage due to the standard battery B9. The latter battery is preferably tapped insteps and constitutes the main control of the high potential output.

The filament of the first amplifier tube` TI is connected to aregulated current. source, whichmay be of the type illustrated in Fig. 8 and hereinafter described, in order to assure constant electron emission. Screen grid potential is obtained from a voltage divider19-8l connected to a suitable source of positive potential which is applied to terminal 83. Plate voltage is obtained from a resistor |85 connected between the plate and the terminal 83. The plate is connected to the control grid of the second amplifier tube 85 through a parallel connected resistor 81 and capacitor 89. A phase control network comprising resistor `9| and capacitor 93 is connected in shunt with the plate of the amplifier tube 11. Screen grid potential for the second amplier tube 85 is derived through a dropping resistor 95 from a suitable source of positive potential which is connected to a terminal 91. A gas iilled regulator tube 99 is utilized to control the screen grid potential. The anode of the output tube 85 is connected through an anode load resistor IUI to the positive supply terminal 83 and also through an isolating resistor |03 to the screen grid of the second oscillator tube V4, which is shown in Fig. 4.

It will be noted that the plate voltage of the first amplifier tube 11 is impressed on the grid of the second amplifier tube 85 through the coupling resistor 81. Since the cathodes of the two amplifier tubes are both operated at ground potential, it is necessary to overcome the effect of the plate voltage on the grid of the second amplier. This is accomplished by connecting a source of negative voltage, for example, 350 volts derived from the voltage supply 3| illustrated in Fig. 1, through an isolating resistor |81 to the control grid of the output tube. This has an effect similar to that of the usual series bucking battery in D.C. amplifiers, but has the advantage that one terminal of the sourceof negative voltage is operated at ground potential, which is not true in the conventional case.

It has also been found that it may be desirable to drive the screen grid of the oscillator tube more negative than is possible by direct control from the amplifier tube 85. This is accomplished by connecting the 350 volt source to thelead |09 through a resistor` thus reducing the average potential of the screen and making it possible to drive the screen more negative when the output tube 85 is drawing maximum plate current.

Protectivev circuit It has been found that, due to the high voltage used, there is some danger of arcing at the high voltage terminals. This may occur Within the vacuum chamber of the microscope, due to a failure of the vacuum, for example, or it may occur by reason of a failure of the insulation of the high voltage condensers.

Referring to Fig. 3, it will be noted that the ground return of the high voltage supply flows through the output current meter A. In order to protect the meter from danger, a protective circuit 62 has been provided which absorbs sudden current surges and limits the current to substantially the normal value. In order to protect the apparatus from sustained overloads, a cutout or overload relay '68 has been provided. The details of the protective circuit and overload relay are illustrated in Fig. 6, to which reference is now made.

Transformer corresponds to the similarly numbered transformer of Fig. 3 which supplies- 68 cycle alternating current to the first high voltage rectier tube VI. The ground return is made through resistors ||1, |l9, I2I, meter A, resistor |23, the` energizing coil of an overload relay |25 and a resistor |21. A neon regulator tube |29 and a pair of condensers |3| and |33 are connected between the output terminals of the meter A and the junction points of the resistors ||1, ||9 and |2|, respectively. The output terminal of the meter A is also connected to ground through a limiting neon tube |35.

The overload relay circuit includes a switch |39 which, in its normal position, makes contact to a grounded termina1 |4|. When actuated by an overload current, the relay armature makes contact to a terminal |43 which is connected to a source of negative voltage, for example, the -350 volts provided bythe power supply 3|. The armature is connected to the grid return of the oscillator tubes of the driver oscillator as illustrated in detail in Fig. 4. The armature is also connected through a push button |41 to the relay coil through a current limiting resistor |49. The same armature is also connected to a voltage divider |5| across a portion of which a neon indicator tube |53 is connected.

A current surge, caused by an arc or breakdown of a high voltage terminal, flows through the limiting resistors H1, ||9 and |2| and charges capacitors |3| and |33. In addition, the voltage across the output meter A is limited by the neon tube |29. The meter is therefore protected from damage. The sustained overload ows through the actuating coil of the relay |25 and connects the armature to the negative potential source, thus removing the normal ground connection and applying a high negative potential to the driver oscillator sufficient to stop its oscillation. Since the driver oscillator provides the high voltage for the high voltage rectifier system, it will be apparent that the relay immediately shuts oli the high voltage supply. At the same time, the relay connects a holding circuit through the push button |41 so that current ows through a resistor |55 and through the relay to hold it in its closed position. It will remain in this position until the circuit is broken by operating the push button |41. If the fault has cleared, the overload relay will open, restoring the high voltage to the rectiiiers. If the fault has not cleared, the high voltage will not relmain on, indicating that the circuit must be checked. When the overload relay is actuated,

the neon output indicator |53 will light, thus providing a visual indication to the operator that the high voltage has been cut off.

Low voltage or current regulators A current regulator of the type preferred for use with the projection and objective coils of the microscope is illustrated in Fig. '7. A conventional rectier |51 and filter |59 supply Ya high voltage to the plate of a triode |6|, the cathode of which is serially connected to ground through the projection or objective coil |63 and current control resistors |65 and |61, the former being adjustable. The voltage drop across the latter resistors is compared to the voltage `oi a standard battery |69 and applied to the grid of a regulating tube |1I, the plate of which is energized by a suitable source of positive potential connected to an input terminal |13 through a plate resistor |15. The plate of this control tube is connected to the grid of the triode |6| through a coupling resistor |11. I

It will be observed that the cathodeA of the triode |6| is positive with respect to ground.

However, the plate of tube I'll cannot become sufficiently less positive than the cathode of tube IBI to provide the necessary bia's for the latter tube under conditions of low output current. In order to supply the proper voltage to the grid of the tube I 6i, it is connected through an isolating resistor |79 to the source of regulated negative voltage, as in the case of the D.C. amplifier described above.

A current regulator' ofsomewhat simpler form which does not require a standard battery is illustrated in Fig. 8. This amplifier is preferably used for the condenser coil of the microscope and may also be used to regulate the filament current of the D.C. amplifier and the control tube Ill of the current regulator illustrated above in Fig. 7. The unit may also be considered as a voltage regulator since the voltage across a constant impedance is constant when the current through it is constant. circuit is employed, for example, in regulating the output of the regulated 800 volt power supply il.

As before, the conventional rectifier and lter S57 and limiting triode ici, the cathode of which is connected to ground through' the electron microscope condenser coils or the regulated filament, as the case may be, and through the control and adjusting resistors IE5 and 161. drop across the latter resistors is compared to a voltage derived from the regulated -350 volts obtained from the power supply 3 i, thus eliminating the standard batteries |69 utilized in the preceding regulator. A particular advantage of this method is that the standard voltage source is operated with one terminal grounded, which is not possible where standard batteries are used in a series circuit. Since the cathode of the triode i6! is positive with respect to ground, as noted above, it may not be necessary to apply the auxiliary negative potential to the grid of this tube so that in this case the plate of the control tube Ill is connected directly to the grid of the triode itl. It will be apparent that in both cases, changes in the current through the ccil and control resistors produces a variable voltage drop across the resistors which is applied to the control tube lll to vary the amplitude of the current in a direction tending to compensate for the changel The measured stability of the power supply system herein described has been found to be well in excess of that'required for stability and definition of an electron microscope. hundred photomicrographs have been successfully taken with no indication that the results have been limited by variations of the power supply. This is in distinct contrast with microscopes .0f the prior art in which a large percentage of the photomicrographs is spoiled by reason of changes in the power supply voltage, The system is so stable that the microscope may be reset to previous conditions without observing the electron image or the focusing. Ex-

posures thus made have good resolution. In ad-y dition, it is possible to makewide variations of' the image intensity without changing the focus.

A particularly severe test which was success- Consequently, the same control H59 are connected to the plate of a The Voltage Several fully passed by the electron microscope operated in conjunction with the power supply and control system of the present invention is that of reducing the vacuum during operation until an internal arc takes place, causing the overload relay to actuate, then restoring the vacuum, and applying the high voltage to obtain the original picture exactly in focus without readjustment. Previously known microscopes could not be treated in this manner and generally had to be taken apart and repaired after an internal ashover due to the severity of the discharge from the filter.

I claim as my invention:

1. An oscillator in which the frequency of oscil lation is determined by the resonant frequency of a series resonant circuit comprising a pair of discharge devices having input and output circuits, the output of one of said tubes being coupled to the input of the other of said tubes, the output of said other tube including in series relation a resistor and said series resonant circuit, and means for applying the output voltage developed across said resistor to the input of said one tube.

2. A device of the character described in claim l which includes inaddi'tion a parallel resonant circuit connected across the output circuit of said other tube for maintaining the Wave shape of the output Voltage of said tube approximately equal to a sine wave.

3. An oscillator in which the frequency of oscillation is determined by the resonant frequency of a series resonant circuit comprising a pair of discharge devices having input and output circuits, the output of one of said tubes being coupled aperiodically to the input of the other of said tubes, the output of said other tube including in series relation a resistor and said series resonant circuit, and means for applying the output voltage developed across said resistor to the input of said one tube.

4. An oscillator in which the frequency of oscillation is determined by the resonant frequency of a series resonant circuit comprising a pair of discharge devices having input and output circuits, the output of one of said tubes being coupled to the input of the other of said tubes, the output of said other tube including in series relation a resistor and said series resonant circuit, and means inductively coupling 'said resistor to the input of said one tube.

5. An oscillator in which the frequency of oscillation is determined by the resonant frequency of a series resonant circuit comprising a pair of discharge devices having input and output circuits, the output of oneof said tubes being coupled aperiodically to the input of the other of said tubes, the output of said other tube including in series relation a resistor and said series resonant circuit, and means inductively coupling said resistor to the input of said one tube.

6. A device of the character described in claim 5 which includes in addition a parallel resonant circuit connected across the output circuit of said other tube for maintaining the wave shape of the output voltage of said tube approximately equal to a sine wave.

ARTHUR W. VANCE. 

